Did Planets from the Inner Disk of the Milky Way Become Rogue Planets?

 We think of a planet as a non-luminous body that orbits a star, like the planets of our solar system. But that’s not always the case.

All stars form from clouds of gas and dust that collapse inward due to the pull of gravity. The pressure caused by the gravitational crunch squeezes gas in the center of each cloud so tightly that it heated the gas to extreme temperatures, generating thermonuclear reactions, and a new star is born. Our sun flared into existence four and a half billion years ago, far younger than the oldest stars, which are born in other parts of the galaxy.

But there was still quite a bit of leftover gas and dust surrounding the young sun which formed a disk around the new star. This leftover bit eventually becomes all of the planets, moons, comets, and asteroids that orbit our sun.

Our solar system lives in the spiral arms of the Milky Way galaxy. Most of the galaxy’s younger stars like our sun are in the spiral arms. Astronomers estimate that virtually all of these stars have planets, an average of two and a half planets per star.

The older stars of our galaxy mostly reside in a bulge surrounding the center of the Milky Way. Stars there have on average barely one planet per star.  MIT astrophysicist Tim Hallatt thinks he knows why. “The puzzle is, these planets (in the spiral arms) are very common,” Hallatt says. “And yet when we look at this other dominant population of stars in the Milky Way, they’re less common. So what’s going on?”

As is the case with all large galaxies like ours, when the Milky Way first formed some 12 billion years ago, star formation was fast and furious, a time Hallatt describes as galactic chaos, what astronomers generally refer to as “cosmic noon.” Also, the stars there were more closely bunched together than the stars in our neighborhood. The greater levels of energetic radiation from the process of rapid star formation plus the relative proximity of stars meant that the stars during cosmic noon experienced ten million times greater levels of radiation. This intense radiation would have heated the gas surrounding all these rapidly forming stars. The greater levels of radiation and heat blew away much of the remaining gas, leaving less raw material for planets.

When astronomers search for planets beyond our own solar system, they look at other stars. We currently know of more than 5000 such exoplanets, and the more we look, the more we find. Astronomers also find lots of rogue planets, planets that don’t orbit any star. They may have formed around a star but were ejected from their home stellar system, perhaps due to close passage of another star. The gravity of the passing star can rip a planet away from its home. Astronomers estimate that perhaps as many as four trillion rogue planets exist in our galaxy alone. That’s a huge number.

Artist's conception of a Rogue Planet

It’s likely that some of those rogue planets formed on their own, not as part of a stellar system. Perhaps some of the gas and dust blown out by the stars formed during the crowded cosmic noon eventually coalesced into rogue planets. Many of these rogue planets could have orbited a star but for the early period of rapid star growth.

Rogue planets may easily outnumber the stars in our galaxy. And some of those rogue stars may be causalities of the cosmic noon timeframe of our Milky Way galaxy.

Each month, I write an astronomy-related column for the Oklahoman newspaper. After it is published there, I post that same column to my blog page.

This is reprinted with permission from the Oklahoman and Oklahoman.com.